1. Field of the Invention
The present invention relates to an automatic exposure control device of a camera, more particularly, the present invention relates to an automatic exposure control device for changing over a size of a stop opening according to subject brightness.
2. Background Arts
Lens-fitted photo film units having pre-loaded photo films are widely sold as one kind of simple cameras. In the lens-fitted photo film unit (hereinafter referred to as film unit), it is unnecessary to load and take out the photo film, so the film unit makes it possible to photograph by simple operation. Moreover, a fixed-focus lens and a shutter mechanism the shutter speed of which is fixed are used in the film unit in order to reduce the manufacturing cost.
The exposure value is constant when the aperture size and the shutter speed are fixed. In severe photographing condition, there is a case in which an exposure amount is out of the latitude of the photo film, so that the photo film is extremely over-exposed. In order to widen the range of subject brightness for proper exposure, it is proposed to provide an automatic exposure control device in the film unit for controlling exposure amount according to subject brightness.
The automatic exposure control device comprises a stop control circuit and a stop changeover mechanism. The stop control circuit drives an electromagnetic driving unit like a solenoid according to subject brightness measured by a light receiving element. The electromagnetic driving unit drives the stop changeover mechanism to change over size of stop opening. In the stop changeover mechanism for changing over size of stop opening by two steps, a large stop opening is set on a photographing optical axis when subject brightness is under the threshold level. A small stop opening is set on a photographing optical axis when subject brightness is equal to or more than the threshold level.
As the light receiving element, a cadmium sulfide (CdS) element is popularly used. The CdS element is low priced, but have dispersion in sensitivity. Moreover, the CdS element has characteristics to vary its resistance widely according to temperature and voltage of power source. Therefore, it is difficult to measure subject brightness precisely.
A photo diode can be used as the light receiving element. The individual variation in sensitivity of the photo diode is less than that of the CdS element. And the photo diode is less influenced by the fluctuation of the temperature and the power voltage (inversely biased voltage). With these reasons, the photo diode makes it possible to measure subject brightness precisely. Furthermore, the response speed of the photo diode is faster than that of the CdS element, so it is preferable to use the photo diode in the case where a momentary photometry is needed.
However, since the photo current of the photo diode is less than 1 xcexcA when subject brightness is around the threshold level, it is difficult to decide precisely whether subject brightness is equal to or more than the threshold level. Accordingly, the photo current of the photo diode is needed to be amplified by an amplifier, such as an OP amp. It causes complication of the photometry circuit and high manufacturing cost. Furthermore, a 3 V to 5 V power supply is necessary to drive the amplifier. Although an R6 battery (1.5 V) is provided in the film unit as a power source of flash device, it is impossible to apply the battery as a power source of the automatic exposure control device.
In view of the foregoing, an object of the present invention is to provide an automatic exposure control device that measures subject brightness precisely and operates with low voltage.
Another object of the present invention is to provide an automatic exposure control device that is capable of being driven by a battery for driving a flash device.
To achieve the above objects, an automatic exposure device of the present invention is comprised of a photo diode for measuring subject brightness, a resistor connected to the photo diode, a field effect transistor (FET) having a gate and a source between which a terminal voltage across the resistor is applied. The FET is turned on when the voltage between the gate and the source is equal to or more than threshold level. The automatic exposure control device controls power to a solenoid according to state of the FET, and changes size of stop opening.
In the preferable embodiment of the present invention, a first transistor is connected to the FET. The first transistor is turned off when the FET is turned on, and is turned on when the FET is turned off. When the first transistor turns on, a second transistor is also turns on. The solenoid is connected to the second transistor. When the solenoid is turned on, a stop plate is retracted from an optical axis of a taking lens, so an exposure is taken through a large stop opening. When the solenoid is turned off, a stop plate is set on the optical axis, so an exposure is taken through a small stop opening.
In the more preferable embodiment of the present invention, the automatic exposure control device comprises a latch circuit for keeping the first transistor turned on. The latch circuit prevents changing over size of stop opening even in a case where the state of the FET changes while exposing. Moreover, the automatic exposure device comprises a capacitor for delaying operation of the first transistor. Since the state of the first transistor is defined after the state of the FET is stabilized, the automatic exposure control device operates stably. Electric power for driving the automatic exposure control device is supplied from a battery for a flash device. The charging operation of the flash device is forced to stop while the automatic exposure control device is in operation. Therefore, it is possible to prevent a faulty operation of the automatic exposure control device due to a decrease of the terminal voltage of the battery.
According to the present invention, since the exposure control device having the photo diode and the FET decides whether a subject brightness is equal to or more than a threshold level, the exposure control device can operate reliably on low voltage. Moreover, the exposure control device of the present invention is provided at a low cost because the configuration of the electronic circuit is simple.